1. Field of the Invention
The present invention relates to a method of automatic frequency assignment for determining frequencies assigned to radio ports constituting a cellular mobile communication system.
2. Description of the Related Art
In a cellular mobile communication system, a frequency is reused repeatedly in a service area for efficient utilization of limited number of frequencies, and the same frequency is assigned to a plurality of radio ports (or radio base stations) that are located distant from one another keeping sufficient distance from each other so as not to have radio interference. It is called frequency assignment to determine a frequency used by each radio port. One approach of frequency assignment is an automatic port frequency assignment (reference "JTC (AIR)/95.06.08-033R3, page 313-315, Annex C--Automatic Port frequency assignment").
A cellular mobile communication system employing the automatic port frequency assignment consists of a radio port control unit (hereinafter called an "RPCU") and a radio port (hereinafter called an "RP") connected to the RPCU. The RP transmits and receives radio signals to and from a personal station (hereinafter called a "PS") by using a frequency assigned to the RP. Adoption of the automatic port frequency assignment makes it easy only by measuring receiving signal power at each RP to re-assign the frequency due to a change of interference signal power being affected, which is caused by environmental changes such as increase of new RPs, buildings, or the like.
FIG. 6 is a flowchart showing a process operation in a conventional automatic port frequency assignment. In the automatic port frequency assignment, an RP to which a new frequency is to be assigned receives transmitted signals from other RPs for each frequency, and reports to an RPCU information correlating to measured received signal power with each frequency (S601). The RPCU selects a frequency that has the lowest received signal power or a frequency with the lowest interference signal power from the reported information (S602), and assigns it to the RP (S603). It performs a similar process for all RPs to assign frequencies to them (S604). The frequency assignment is achieved by repeating the operation of steps S601-S604 for a predetermined number of times to reduce the effect of error (S605).
In the method of automatic port frequency assignment just described, a frequency with the lowest received signal power from other RPs is assigned to the RP. To this end, as shown in FIG. 7, if a shield such as a building exists between the RP 101 to which a frequency is to be assigned and the RP 102 adjacent to the RP 101, the received signal power from the RP 102 becomes lowered, so that the RP 101 would be assigned with the same frequency as that of the RP 102. If the same frequency is assigned to the RP 101 and the RP 102, in the actual service, when the PS 103 being communicated with the RP 102 approaches the RP 101, the PS 103 receives a down link signal from the RP 101 as an interference signal, so that the communication quality of down link at the PS 103 is degraded. In addition, since the RP 101 receives an up link signal from the PS 103 as an interference signal, the communication quality of up link at the RP 101 is also degraded.
In addition, since the automatic port frequency assignment described above is a method for assigning a frequency taking into account only interference signal power on the down link transmitted by the RPs, it is impossible to avoid degradation of the communication quality on the up link.